feat(docs): add pinned copy-to-clipboard button for code blocks

[shekhar-narayan-mishra]

Summary

This PR adds a copy-to-clipboard button to all

 blocks in the Deno documentation.

Previously, users had to manually select and copy code snippets, which was inefficient and error-prone — especially for longer commands. This enhancement improves usability by adding a persistent, accessible copy button that works across all code blocks.
Features
✨ Adds a copy button pinned to the top-right corner of every code block
✨ Handles horizontal scrolling: button stays fixed while code scrolls
✨ Strips shell prompts like $, >, and ❯ before copying
✅ Displays a ✔️ check icon briefly after successful copy
✅ Styled to blend with the existing docs look (no emojis, clean UI)
Implementation
copy.client.ts: injects the copy button and handles copy logic
styles.css: includes styles for button positioning, appearance, and animation
Code is scoped and non-invasive; it de-duplicates any existing theme-based buttons
How to Test Locally
Run the dev server:
deno task serve
Visit any page with code examples (e.g. Install or Getting Started)
Hover over a code block — confirm:
Copy icon is pinned to top-right
It stays fixed during horizontal scroll
Clicking it copies the code (without $, >, etc.)
A check icon appears briefly then reverts
Linked Issue:
Closes #2745
Screenshots
Before:

After:

Notes for Reviewers
This is a non-breaking, client-side UX improvement
Does not modify build, runtime, or API logic
Happy to refine styling or placement if requested

[bartlomieju]

Thanks for the contribution! However, the docs site already has a copy-to-clipboard button on all code blocks. It's implemented as a build-time markdown-it plugin (markdown-it/codeblock-copy.ts) that injects a copy button into every fenced code block, with the click handler in js/copy.ts.

The existing approach is preferable since the button is available immediately on page load (no DOMContentLoaded delay) and the copy text is captured from the raw markdown at build time, avoiding any issues with scraping rendered DOM content.

Closing as the feature already exists. If there's a specific issue with the current copy button (not appearing on certain pages, etc.), feel free to open a new issue describing the problem!